全文获取类型
收费全文 | 6053篇 |
免费 | 245篇 |
国内免费 | 49篇 |
专业分类
化学 | 2734篇 |
晶体学 | 159篇 |
力学 | 143篇 |
数学 | 667篇 |
物理学 | 2644篇 |
出版年
2023年 | 54篇 |
2022年 | 51篇 |
2021年 | 61篇 |
2020年 | 92篇 |
2019年 | 67篇 |
2018年 | 76篇 |
2017年 | 91篇 |
2016年 | 150篇 |
2015年 | 182篇 |
2014年 | 165篇 |
2013年 | 421篇 |
2012年 | 258篇 |
2011年 | 292篇 |
2010年 | 315篇 |
2009年 | 315篇 |
2008年 | 239篇 |
2007年 | 285篇 |
2006年 | 319篇 |
2005年 | 228篇 |
2004年 | 210篇 |
2003年 | 206篇 |
2002年 | 184篇 |
2001年 | 191篇 |
2000年 | 139篇 |
1999年 | 195篇 |
1998年 | 105篇 |
1997年 | 102篇 |
1996年 | 83篇 |
1995年 | 67篇 |
1994年 | 70篇 |
1993年 | 52篇 |
1992年 | 40篇 |
1991年 | 30篇 |
1990年 | 39篇 |
1989年 | 39篇 |
1988年 | 142篇 |
1987年 | 190篇 |
1986年 | 173篇 |
1985年 | 49篇 |
1984年 | 78篇 |
1983年 | 34篇 |
1982年 | 59篇 |
1981年 | 45篇 |
1980年 | 40篇 |
1979年 | 30篇 |
1978年 | 43篇 |
1977年 | 20篇 |
1976年 | 23篇 |
1975年 | 4篇 |
1936年 | 1篇 |
排序方式: 共有6347条查询结果,搜索用时 15 毫秒
101.
Ogawa Y Ogawa K Kokufuta E 《Langmuir : the ACS journal of surfaces and colloids》2004,20(7):2546-2552
Polyampholyte gels were prepared by free radical polymerization of aqueous monomer solutions with the following composition: 69% N-isopropylacrylamide (thermosensitive neutral monomer), 1% N,N'-methylenebisacrylamide (cross-linker), 15% 1-vinylimidazole (cationic monomer), and either 15% acrylic acid (AAc, anionic monomer) or poly(acrylic acid) (PAAc, polyanion). We thus obtained two sorts of polyampholyte gels; that is, G1 with immobilized PAAc and G2 with randomly copolymerized AAc. The equilibrium swelling ratio (Qe) was studied as a function of the pH, NaCl concentration, and temperature. Also studied was the kinetics of swelling and shrinking in response to a sudden pH change. The significant results obtained were as follows: (i) A fully collapsed state was observed at pH 4.5-9.0 for G1 and at pH 4.5-7.0 for G2. (ii) Below and above these pH ranges, both gels were in a swollen state; therefore, an isoelectric point (pI) appeared in a wide pH range. (iii) At alkaline pH regions where a hysteresis was observed in the Qe versus pH curves of G1 and G2 as the pH was first increased then decreased, G1 exhibited very slow swelling-shrinking kinetics. (iv) An increase in the NaCl concentration allowed the gel to swell at pH approximately pI (antipolyelectrolyte behavior) but to shrink at pHs below and above the pI range (polyelectrolyte behavior). (v) The magnitude of the salt-induced shrinking of G1 is smaller than that of G2 at pH 10 and at NaCl concentrations > 0.01 M. (vi) At pH 10, an increase in the temperature from 35 to 50 degrees C led to a shrinking change of G1 but not of G2. These results were found to be explicable in terms of a different distribution of negative charges within the polyampholyte gel network. 相似文献
102.
《Journal of photochemistry and photobiology. A, Chemistry》2006,177(2-3):170-176
Novel photochromic benzimidazol[1,2a]pyrrolidin-2-ones, which give thermally stable highly coloured photochromes, have been synthesised by condensation of 2-methyl-3-benzothienylethylidene(isopropylidene)succinic anhydride with 1,2-diaminobenzene and its 4,5-dimethyl and 4,5-dimethoxy derivatives. The photochromic properties are reported. 相似文献
103.
A kinetic study on enzymatic S-oxygenation of sulfoxides to sulfones was carried out by a reconstited system with purified cytochrome P-450. A linear correlation observed between log(Vmax)'s and the one-electron oxidation potentials of sulfoxides suggests that the oxygenation of sulfoxides proceeds via one electron transfer process to the active “oxenoid” intermediate of the enzyme. 相似文献
104.
Cao B Wakahara T Tsuchiya T Kondo M Maeda Y Aminur Rahman GM Akasaka T Kobayashi K Nagase S Yamamoto K 《Journal of the American Chemical Society》2004,126(30):9164-9165
A new metallofullerene, La2@C78, has been synthesized by DC arc discharge method, isolated by high-performance liquid chromatography, and characterized by laser desorption time-of-flight mass spectrometry, UV-vis-NIR absorption, differential pulse voltammetry, 13C NMR spectroscopy, and theoretical calculations. The La2@C78/CS2 solution is dark violet and presents several characteristic absorption features at 647, 561, 533, and 386 nm, with an onset around 1000 nm. With respect to empty D3-C78, the capability of La2@C78 as an electron acceptor or donor is stronger. Addition of 1,1,2,2-tetrakis(2,4,6-trimethylphenyl)-1,2-disirane to La2@C78 photochemically, as well as thermally, affords bis- and mono-adducts. Theoretical studies and 13C NMR spectroscopic analysis of La2@C78 indicate that it possesses a D3h-C78 cage (78:5). 相似文献
105.
106.
Subratanath Koner Masaru TsutakeIsoroku Nagasawa Ryuichi IkedaPratap K. Saha Alok K. MukherjeeSurajit Banerjee 《Journal of Molecular Structure》2002,608(1):63-69
The complex bis-(N,N-dimethyl-1,2-ethanediamine) nickel(II) perchlorate undergoes a first-order thermochromic phase transition at ca. 476 K, changing its color from orange to red. The room temperature X-ray crystal structure determination showed that the nickel ion possesses a square-planar geometry with two five membered chelate rings, in the δλ conformation, forming the NiN4 chromophore. The broad-line 1H NMR indicates the onset of a dynamic disorder of diamine chelate rings at the phase transition temperature region, while T1 measurement of 1H affords the activation energy of the puckering metal chelate rings to be 26 kJ mol−1. The electronic spectrum revealed that a weakening of ligand field around the nickel is associated with the phase transition. 相似文献
107.
Nagao S Hirai Y Suzuki A Yamamoto Y 《Journal of the American Chemical Society》2005,127(12):4146-4147
A 19F NMR study on the acid-alkaline transition in sperm whale myoglobin reconstituted with a perfluoromethyl heme, 13,17-bis(2-carboxylatoethyl)-3,8-diethyl-2,12,18-trimethyl-7-trifluoromethylporphyrinatoiron(III), demonstrated that the thermodynamics of the transition is predominantly controlled by the stability of acidic form. 相似文献
108.
Kotaro Hidea Hiroshi Maezawa Atsushi Ito Katsumi Kobayashi Yoshiya Furusawa Takashi Ito 《Photochemistry and photobiology》1986,44(3):417-419
The spectral throughput of a vacuum-ultraviolet irradiation system at the SOR-RING facility was examined with various combinations of aluminum-and gold-coated optical elements in a 2.2-m modified Wadsworth monochromator. We found that the optimum was a combination of an aluminum-coated collimating mirror, concave grating, and plane deflecting mirror, and a combination of a gold-coated collimating mirror, concave grating and an aluminum-coated plane deflecting mirror in the wavelength regions 190-110 nm and 110-50 nm, respectively. 相似文献
109.
Sulphoximides (Ia–Ie) were found to react with dimethyl diazomalonate (DDM) in the presence of a catalytic amount of Cu-salts affording the corresponding oxosulphonium ylides in moderate yields. The reaction did not proceed at all under irradiation of UV light. (?)-Methylphenyloxosulphonium bis(methoxycarbonyl)-methylide ((?)-IIb) was obtained from (+)-(S)-methylphenylsulphoximide ((+)-(S)-Ib) together with (?)-(S)-methyl phenyl sulphoxide ((?)-(S)-IIIb) by this reaction. Hydrolysis of (?)-IIb gave (+)-methylphenyloxosulphonium methoxycarbonylmethylide ((+)-IIf) which was converted to (?)-(S)-IIIb upon treatment with dibenzoylethylene. Stereochemical cycle starting from (+)-(S)-Ib to (?)-(S)-IIIb was established and the absolute configurations of both ylides, (?)-IIb and (+)-IIf were assigned as (R)-configuration. The stereochemical courses, namely from (+)-(S)-Ib or (?)-(S)-IIIb to (?)-(R)-IIb or (+)-(R)-IIf to (?)-(S)-IIIb were determined as retention processes. The optical purities of the oxosulphonium ylides obtained from both reactions, (+)-(S)-Ib→(?)-(R)-IIb and (?)-(S)-IIIb→(?)-(R)-IIb, were almost equal. These results indicate that the mechanism of the reaction of sulphoximides with carbenes (or carbenoids) involves the initial formation of the sulphoxides which react subsequently with carbenes to afford the final products. 相似文献
110.
Asymmetric catalytic hydrogenations of three kinds of chiral pyruvamides were carried out using palladium on charcoal as a catalyst to give lactamides with diastereomeric ratios ranging from 76:24 to 98:2. 相似文献